478 A. G. Bell—Production of Sound by Radiant Energy. 
turned out to be the case, and the importance of the re 
is very great, especially when we consider the expense of suc 
rare substances as selenium and iellurium. 
The form of lamp-black cell we have found most effective is 
shown in fig. 8. Silver is deposited upon a plate of glass, 
and a zigzag line is then scratched through the film, as shown, 
1 | 
dividing the silver surface into two portions insulated from one 
another, having the form of two combs with interlocking teeth. 
ach comb is attached to a screw-cup, so that the cell can be 
placed in an electrical circuit when required. The surface is 
then smoked until a good film of lamp-black is obtained, filling 
the interstices between the teeth of the silver combs. When 
the lamp-black cell is connected with a telephone and galvanic 
battery, and exposed to the influence of an intermittent beam 
of sunlight, a loud musical tone is produced by the telephone. 
This result seems to be due rather to the physical condition 
than to the nature of the conducting material employed, as 
metals in a spongy condition produce similar effects. For in- 
stance, when an electrical current is passed through spongy 
platinum while it is exposed to intermittent sunlight, a dis- 
tinct musical tone is produced by.a telephone in the same cir- 
cuit. In all such cases the effect is increased by the use of an 
induction coil; and the sensitive cells can be employed for the 
i 
1 
itl 
i 
ut 
ta! 
Hii 
